1. Field of the Invention
The present invention relates to lens element, a lithographic apparatus, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA (when supported by the lens) of the system and also increasing the depth of focus.) Several immersion liquids have been proposed.
In the case of the immersion lithography in which the area between the substrate and a last lens of the projection system is filled with liquid, the lens numerical aperture (NA) is limited to one which is dependent on its refractive index (even if the refractive index of the liquid is greater than that of the lens) in order to prevent total internal reflection of the projected beam as it passes from the lens to the liquid on its path to the substrate. This limits the angles in which the projected beam can pass safely through the lens and the liquid to the substrate. One way to overcome this limitations is to make the lens out of a material with a larger refractive index, but none greater than n=1.56 is confirmed as being suitable for use at the time of writing.
Another way to overcome the limitation of the angles in which the projected beam can pass safely through the lens and the liquid to the substrate is described in a patent application filed as a U.S. patent application Ser. No. 10/959,403, filed Oct. 7, 2004, in the name of the applicant of this application. The document proposes to provide a curved lens element, in which the lens-liquid boundary is curved by using a curved lens element. This enables a numerical aperture not to be limited by the refractive index of the material, but by the curvature of the surface of the lens. However, by providing a concave lens-liquid boundary, the distance between the curved lens element and the substrate is increased. Particularly in the centre of the curved lens element the distance between the curved lens element and the substrate may be a factor ten larger compared to a substantially flat lens-liquid boundary.
In case of immersion lithography in which the area between the substrate and a last lens (closest to the substrate) of the projection system is filled with liquid, this increased lens-substrate distance may cause problems, as the optical path of the radiation beam through the liquid is increased, considerably reducing the intensity of the radiation beam at the substrate level. Also, the arrangement relatively takes up a lot of space.